The invention relates generally to Wheatstone bridge circuits used in sensors for monitoring an applied stimulus such as pressure, force, acceleration and the like in automotive safety critical applications and more particularly to ASICs (application specific integrated circuits) having a simple architecture to enable low cost sensors for such applications.
A sensor of the type described herein is used, for example, in an automotive application for monitoring an applied stimulus, such as brake fluid pressure, force, acceleration and the like and, as in the case of a pressure sensing application, typically comprise a sense element to provide a Wheatstone bridge signal in response to a target stimulus. By way of example, a pressure sensing sense element can be comprised of four piezoresistors bonded to a diaphragm having appropriate interconnectivity and diaphragm placement. The sensor typically includes a sense element, a circuit (e.g., ASIC) which amplifies and compensates that signal which then is typically inputted through a wiring harness or the like into an analog to digital converter of an Electronic Control Unit (ECU), for example, for further processing.
Many safety critical applications such as automotive electro-hydraulic braking, brake assist, inertial stability systems having braking control and the like, depend on the validity of in-range sensor signals. In-range sensor signals are signals within the normal range of output signals over all application tolerances and stimulus range limits. Conventionally, safety critical systems have relied on having two sensors with agreement of the sensor signals providing assurance of the validity of the signals. Although this approach provides suitable assurance of signal validity, it is a costly solution requiring an extra sensor, wire harness and connector.
Another approach has been to add extensive diagnostic circuits in the sensor""s ASIC to provide a higher degree of reliability for in-range sensor outputs compared to a similar single sensor not having such diagnostic circuits. This approach provides a cost savings over the two sensor approach, however, this approach requires the use of extensive ASIC die area thereby adversely impacting the sensor""s cost, complicates the calibration algorithm and does not guarantee signal validity for signals in the active range for all plausible fault conditions. For example, the single output format of such a sensor does not detect relatively high occurrence failure modes which can invalidate in-range output signals. By way of example, such failures could occur due to an electrically damaged output stage or excessive connector resistance developed during long driving periods of an automobile having such a sensor.
It is an object of the invention to provide a conditioning circuit useful in an ASIC which overcomes the limitations of the prior art mentioned above. Another object is the provision of a simple, low cost conditioning circuit useful with a sensor comprising a Wheatstone bridge having piezoresistive bridge elements which enables improved diagnosis of the sensor signal. Yet another object is the provision of such a circuit which can provide a temperature output, or redundant temperature outputs at selected times such as during a start-up phase of the sensor.
Briefly in accordance with the invention, a Wheatstone bridge circuit having applied stimulus sensitive piezoresistive bridge elements comprises independent signal conditioning paths for each half bridge. Each half bridge is connected between an independent temperature dependent bridge supply voltage and ground. The output of each half bridge is connected to one input of a respective amplifier and a selected proportion of the temperature dependent bridge supply voltage is connected to a second input of the respective amplifier.
The temperature dependent Wheatstone half bridge supply voltages are also inputted to respective offset and gain compensation control circuits which provide temperature compensation of the transfer function of the amplifiers. The outputs of the half bridges are a pair of voltage output signals which are mutually proportional to the supply voltages and stimulus. The two half bridge outputs have nominally equal stimulus sensitivities of opposite sign.
According to one embodiment, the output of the first amplifier connected to the first half bridge is connected to one of two inputs of a third amplifier with the other input of the third amplifier receiving the output of the second amplifier connected to the output of the second half bridge. The output of the second amplifier is also connected to the single input of a fourth amplifier so that the output of the third amplifier provides a signal proportional to the Wheatstone bridge differential output voltage and the output of the fourth amplifier provides a signal proportional to the Wheatstone half bridge output voltage.
According to a second embodiment, the output of each half bridge is connected to the single input of an amplifier having a fixed functional relationship to provide two output signals, each proportional to a respective Wheatstone half bridge output voltage.
According to yet another embodiment, the temperature dependent voltage signal and the signal proportional to the Wheatstone half bridge output voltage are multiplexed to the single input of an output amplifier under the control of a state machine, or the like, to provide a temperature dependent signal on an output node of the ASIC for a selected time period followed by the signal proportional to the Wheatstone half bridge output on the same output node on a continuous basis after the expiry of the time period.
The above embodiments enable detection of plausible sensor or sensor interconnectivity flaws from the sensor""s sense element to the ECU (electronic control unit) input signal measurement circuit in a robust yet simple manner enabling the provision of an improved low cost sensor.